We will critically test the hypothesis of selective neutrality of naturally occuring electrophoretic enzyme varients using Escherichia coli in chemostats. We are studying the full range of naturally occurring allelic variation at gnd (6-phosphogluconate dehydrogenase). Other loci will also be looked at, possibly mdh (malate dehydrogenase). By studying competition between siogenic strains in chemostats, we empirically define physiologically stressful environments for each locus as those in which selection between plus and minus strains is maximal (minus strain will be mutants having no active enzyme); similarily physiological nonstressful envionments will be empirically defined as those in which selection between plus and minus strains is minimal. Pairs of electrophoretic variants made effectively isogenic by transduction will then be competed in chemostats in both stress and nonstress environments, and the selection coefficients estimated. The finding of no selection, or drastically reduced selection, in nonstress environments serves to control against the possibility that the selection is due to loci closely linked to the tested locus. The empirical distribution of selection coefficients of naturally occurring varients will then be compared with the distribution of selection coefficients of new (created in the laboratory and unselected) electrophoretically detectable varients.